For many years, both direct current (DC) and alternating current (AC) power lines have been used in order to transfer data from one device to another. Recently, there has been a growing need for new data transmission services and applications that are more reliable and support higher data rates over these power lines. For instance, remote metering, smart grids, industrial and home automation are merely some of the upcoming applications that are currently using power lines to support data communications and greater use of these power lines is expected.
One primary disadvantage in using power lines for data transfer is that power lines are hostile environments. In fact, communication channels supported by these AC power lines tend to experience severe non-linear behavior. In particular, channel characteristics and parameters may vary due to changes in frequency, location, time and even the type of equipment deployed. As an example, the impedance of a power line may appear to be 1-2 ohms (Ω), but as the frequency of signaling applied to the power line increases, the impedance of the power line also increases. This increased impedance causes increased signal noise that may hamper proper detection of the data at an intended destination.
FIG. 1 illustrates an exemplary noise power distribution 100 with frequency bands 110-112 supported by European (CELENEC), United States (FCC) and Japan (ARIB) power line standards respectively, as outlined in Table 1.
TABLE 1LOW FREQHIGH FREQSTANDARDS(KHz)(KHz)FCC10480ARIB10450CELENEC A995CELENEC B95125CELENEC C125140CELENEC B, C95140
As illustrated in FIG. 1, a low frequency region from three kilohertz (3 kHz) to 500 kHz is especially susceptible to interference such as narrowband interference and/or intersymbol interference (ISI), which may occur if orthogonal frequency division multiplexing (OFDM) is used as the selected data transmission scheme.
OFDM is a multi-carrier modulation scheme that subdivides the available frequency spectrum into a number of narrowband channels (e.g., around 100 channels). The carriers for each channel may be spaced much closer together than Frequency Division Multiplexing (FDM) for example, because each carrier is configured to be orthogonal to its adjacent carriers. This orthogonal relationship may be achieved by setting each carrier to have an integer number of cycles over a symbol period. Hence, theoretically, there is no interference between the carriers themselves, albeit interference caused by environmental conditions may exist.
Besides interference, the low frequency region is highly susceptible to impulsive noise and group delay. “Impulsive noise” is characterized as a short peak pulse substantially less than one second (e.g., a few microseconds) and with a sharp rise time above the continuous noise level experienced by the signal. “Group delay” is a measure of the rate of change of the phase with respect to frequency. As an effect of non-constant group delay, phase distortion will occur and may interfere with accurate data recovery.
Prior power line communications have virtually avoided using OFDM-based communication techniques within the lower frequency region. One reason for such avoidance can be ascertain by review of the noise power distribution of FIG. 1. Between 10 kHz and 148 kHz (Celenec bands shown as region A 110), the effects of noise on an input signal of a given frequency level is about 10-30 decibels (dB) stronger than the noise experienced by signals at approximately 250 kHz as represented in region B 120. Moreover, the channel frequency response varies drastically within the region between 10 kHz-150 kHz, resulting in severe amplitude and phase distortion.